Actions of activins, inhibins and follistatins: implications in anterior pituitary function

1. The anterior pituitary is well documented to be under the control of central and peripheral factors that dynamically interact to affect cell-specific modulation of pituitary functions. However, it is becoming increasingly evident that these extrinsic factors work in concert with a variety of local products that exert autocrine/paracrine control on pituitary cells. 2. These factors modulate the activity of their target pituitary cells by altering the synthesis and secretion of cell-specific hormones and by exerting control on the growth and differentiation of cells of this tissue. Included in the list of growth factors and bioactive peptides known to be products of pituitary cells are the activins, possibly inhibins and follistatins. 3. These protein factors play an important role in the local modulation of several pituitary cell types and are crucial for the maintenance of normal follicle-stimulating hormone production and, thus, reproductive function and fertility.

Regulation and actions of Smad7 in the modulation of activin, inhibin, and transforming growth factor-beta signaling in anterior pituitary cells

Activins and transforming growth factor-beta (TGF beta) are crucial autocrine, paracrine, and endocrine modulators of anterior pituitary function. Activins regulate most pituitary cells and lactotropes are targets of TGF beta. Smad2 and Smad3 are two cellular mediators of activin/TGF beta signaling, whereas Smad7 is as an inducible, negative modulator of the pathway. This study was undertaken to evaluate Smad7 regulation in the pituitary. Activin A rapidly and transiently increased Smad7 messenger RNA (mRNA) levels of rat anterior pituitary (RAP), clonal gonadotrope (alpha T 3-1 and L beta T2), and corticotrope (AtT20) cells with an EC(50) of 0.1-0.2 nM. In RAP cells, activin A or TGF beta 1 had equivalent effects that were additive. Follistatin, known to bind and inactivate activins, prevented Smad7 induction by activin. Inhibin A partially antagonized activin A, perhaps reflecting gonadotrope-selective actions. This antagonism was also evident with alpha T 3-1 and L beta T2 gonadotropes. Forskolin had no measurable effect in RAP cells, but increased Smad7 mRNA levels in alpha T3-1 cells and decreased them in L beta T2 cells. Transient transfection of Smad7 along with 3TPLux, an activin/TGF beta-responsive reporter, blocked activin-mediated promoter activation in alpha T3-1 and AtT20 cells. In alpha T3-1 cells, which express endogenous follistatin mRNA, a follistatin-luciferase reporter, rFS(rin3)-Luc, was transcriptionally activated by activin A, or when cotransfected with a constitutively active ActRIB [Alk4(T>D)], Smad2, or Smad3. Smad7 blocked rFS(rin3)-Luc activation by activin A or Alk4(T>D). Together, these results point to a role of Smad7 in modulating activin/TGF beta signaling in the pituitary.

Betaglycan binds inhibin and can mediate functional antagonism of activin signalling

Activins and inhibins, structurally related members of the TGF-beta superfamily of growth and differentiation factors, are mutually antagonistic regulators of reproductive and other functions. Activins bind specific type II receptor serine kinases (ActRII or IIB) to promote the recruitment and phosphorylation of the type I receptor serine kinase, ALK4 (refs 7-9), which then regulates gene expression by activating Smad proteins. Inhibins also bind type II activin receptors but do not recruit ALK4, providing a competitive model for the antagonism of activin by inhibin. Inhibins fail to antagonize activin in some tissues and cells, however, suggesting that additional components are required for inhibin action. Here we show that the type III TGF-beta receptor, betaglycan, can function as an inhibin co-receptor with ActRII. Betaglycan binds inhibin with high affinity and enhances binding in cells co-expressing ActRII and betaglycan. Inhibin also forms crosslinked complexes with both recombinant and endogenously expressed betaglycan and ActRII. Finally, betaglycan confers inhibin sensitivity to cell lines that otherwise respond poorly to this hormone. The ability of betaglycan to facilitate inhibin antagonism of activin provides a variation on the emerging roles of proteoglycans as co-receptors modulating ligand-receptor sensitivity, selectivity and function.

Identification of a binding site on the type II activin receptor for activin and inhibin

Type II activin receptors (ActRII and ActRIIB) are single-transmembrane domain serine/threonine kinase receptors that bind activin to initiate the signaling and cellular responses triggered by this hormone. Inhibin also binds type II activin receptors and antagonizes many activin effects. Here we describe alanine scanning mutagenesis of the ActRII extracellular domain. We identify a cluster of three hydrophobic residues (Phe(42), Trp(60), and Phe(83)) that, when individually mutated to alanine in the context of the full-length receptor, cause the disruption of activin and inhibin binding to ActRII. Each of the alanine-substituted ActRII mutants retaining activin binding maintains the ability to form cross-linked complexes with activin and supports activin cross-linking to the type I activin receptor ALK4. Unlike wild-type ActRII, the three mutants unable to bind activin do not cause an increase in activin signaling when transiently expressed in a corticotroph cell line. Together, our results implicate these residues in forming a critical binding surface on ActRII required for functional interactions with both activin and inhibin. This first identification of a transforming growth factor-beta family member binding site may provide a general basis for characterizing binding sites for other members of the superfamily.

Urocortin messenger ribonucleic acid: tissue distribution in the rat and regulation in thymus by lipopolysaccharide and glucocorticoids

Urocortin (Ucn), a new mammalian member of the CRF family, is a candidate endogenous ligand for type 2 CRF receptors. In a survey of peripheral tissues from adult male rats, we found that Ucn messenger RNA (mRNA) was abundant in the gastrointestinal tract and immune tissues such as thymus and spleen. We next tested the hypothesis that levels of Ucn mRNA levels in thymus and spleen would be altered after immune activation. As measured by ribonculease protection assay, lipopolysaccharide (LPS) induced a 2-fold time-dependent increase in thymic Ucn mRNA levels within 6 h. By contrast, splenic Ucn mRNA levels decreased after LPS. Because LPS activates the hypothalamus-pituitary-adrenal (HPA) axis, we examined whether the effects of LPS on Ucn mRNA might be mediated through changes in HPA axis hormones. Ucn mRNA in thymus, but not spleen, was significantly increased after ACTH injection; however, LPS did not increase Ucn expression in the thymus of adrenalectomized rats with corticosterone replacement, despite substantial increases in ACTH. Finally, sc injection of corticosterone stimulated Ucn mRNA comparably to that of LPS. Together, these results suggest that Ucn mRNA expression can increase after immune activation in a corticosterone-dependent manner, and that such changes in Ucn mRNA may be an additional consequence of HPA axis activation.

Interleukin-1beta regulates pituitary follistatin and inhibin/activin betaB mRNA levels and attenuates FSH secretion in response to activin-A

Activins and follistatins regulate all levels of the reproductive axis, including the pituitary where they stimulate and inhibit FSH production, respectively. Gonadotropes are known to express inhibin/activin betaB and activin-B (betaBbetaB) functions as an autocrine modulator of FSH production. By contrast, the mRNA for the activin-binding protein, follistatin, is present in most pituitary cells and folliculo-stellate cells may be the major source of the protein secreted by the anterior pituitary. Interleukin-1beta (IL-1beta) is one of several cytokines known to also influence the reproductive axis. IL-1beta inhibits the hypothalamo-pituitary-gonadal (HPG) axis by suppressing GnRH and gonadal steroid production. Because several pituitary cell types, including follistatin-producing folliculo-stellate cells, are targets of IL-1beta, cytokine effects on gonadotrope function were evaluated using cultured rat anterior pituitary cells. Activin-A (0.01 to 1 nM; 24h) increased basal FSH secretion approximately 2-fold. IL-1beta (0.005 to 0.5 nM) by itself had no effect on basal FSH secretion. However, IL-1beta attenuated FSH secretion in response to all concentrations of activin-A. These results suggest that the cytokine might stimulate the local production of a factor, such as follistatin, that antagonizes the action of activin-A. RNase protection analysis indicated that IL-1beta (0.005 to 5 nM) stimulated follistatin and inhibin/activin betaB mRNA accumulation in a time-dependent manner. These in vitro effects of IL-1beta were blocked by the specific IL-1 receptor antagonist (IL-lra) and were not mimicked by either rhIL-6 or lipopolysaccharide (LPS). Treatment of intact male rats with LPS (50 microg, i.v.), which increases plasma IL-1beta and induces IL-1beta expression in many tissues, including the pituitary, produced similar time-dependent increases in pituitary follistatin and inhibin/activin subunit mRNA levels. These results suggest that IL-1beta can modulate gonadotrope responses to activins by influencing the local balance of activin-B and follistatin within the pituitary.

Pituitary follistatin and inhibin subunit messenger ribonucleic acid levels are differentially regulated by local and hormonal factors

Follistatins, activins, and inhibins are expressed in a wide range of tissues where they function as autocrine and/or paracrine factors. Activin B (beta B beta B) and inhibin B (alpha beta B) are the predominant forms expressed in the rat anterior pituitary. This study was designed to evaluate the regulation of the messenger RNAs (mRNAs) for inhibin alpha and beta B, and follistatin, relative to each other, using cultured rat anterior pituitary cells. Activin A stimulated follistatin (a maximal 4-fold stimulation by 6 h) and beta B (a maximal 1.7-fold stimulation after 2 h) mRNA levels. Although inhibin A dramatically decreased follistatin mRNA levels (34% of the control value after 24 h), it only marginally affected those of beta B (86% of the control value after 2 h). Follistatin inhibited the accumulation of its own mRNA (46% of the control value after 6 h), but had no statistically significant effect on beta B or alpha mRNA levels. Inhibin A was the only treatment that had an effect on alpha mRNA levels, causing a slight decrease (82% of the control value by 24 h). The effects of activin A and inhibin A on follistatin and beta B mRNA levels were dose dependent. Moreover, follistatin and inhibin A blocked the effects of activin A. Immunoneutralization experiments were performed to determine whether locally secreted activin B regulates the expression of these three mRNAs. A monoclonal antibody to activin B reduced follistatin and beta B mRNA levels (37% and 73% of the control value, respectively) and enhanced the stimulatory effect of exogenous activin A on these mRNAs (840% vs. 300% and 170% vs. 145% of the control value, respectively); there was no change in alpha mRNA accumulation. GnRH and activators of the protein kinase A (forskolin) and protein kinase C (12-O-tetradecanoylphorbol acetate) pathways also had differential effects on follistatin, beta B, and alpha mRNA levels. GnRH stimulated follistatin mRNA levels, but suppressed those of beta B. 12-O-Tetraphorbol acetate had no effect on beta B, but stimulated follistatin mRNA levels to the same extent as forskolin. Of these agents, only forskolin produced a marginal inhibitory effect on alpha mRNA accumulation. Testosterone decreased both follistatin and beta B mRNA levels without affecting those of alpha. The results of this study demonstrate that the local production of rat anterior pituitary follistatin, activin B, and inhibin B is regulated by hypothalamic, peripheral, and local factors in such a way that the ratios between activin B and its two inactivators, follistatin and inhibin B, are very tightly maintained.

Corticotropin-releasing factor (CRF) and glucocorticoids modulate the expression of type 1 CRF receptor messenger ribonucleic acid in rat anterior pituitary cell cultures

Previous studies involving radioreceptor and functional assays have shown that CRF and glucocorticoids are able to modulate CRF receptors of the brain and anterior pituitary. In this study, we analyzed the effects of CRF, vasopressin (AVP), dexamethasone (DEX), and corticosterone on the regulation of CRF receptor (CRF-R1) messenger RNA (mRNA) levels in cultured rat anterior pituitary cells. CRF decreased CRF-R1 mRNA levels in a time- and concentration-dependent manner. In the presence of 10 nM CRF, CRF-R1 mRNA levels decreased within 1 h (to 65 +/- 3% of the control value; P < 0.01) with a maximal effect after 3 h (to 28 +/- 1% of the control value; P < 0.001). The concentration dependence of the inhibitory effect of CRF at 3 h correlated with that required for ACTH secretion (half-maximal at approximately 0.03 nM). Treatment with a maximal (100 nM) dose of AVP or a submaximal (0.1 nM) dose of CRF for 3 h reduced CRF-R1 mRNA levels to 66 +/- 3% and 53 +/- 6% of the control value, respectively. In the presence of both AVP and CRF, CRF-R1 mRNA levels were 32 +/- 3% of the control value. The incubation of cells for 3 h with 10 microM forskolin to activate adenylate cyclase or with 20 nM 12-0-tetradecanoylphorbol-13-acetate to activate protein kinase C resulted in a decrease in receptor mRNA levels to 40 +/- 9% (P < 0.01) and 28 +/- 8% (P < 0.001) of the control value, respectively, suggesting that the effects of CRF and AVP may be mediated by these pathways. DEX (20 nM) also caused a dose- and time-dependent decrease in mRNA levels. Maximal inhibition was observed after 3 h (to 31 +/- 6% of the control value; P < 0.001), with a partial recovery of mRNA levels at 24 or 48 h. Corticosterone similarly inhibited the accumulation of CRF-R1 mRNA in a dose- and time-dependent manner, but, in contrast to DEX, CRF-R1 mRNA levels returned almost to control levels after 24 h. These results indicate that the ability of CRF, AVP, and glucocorticoids to modulate the responses of corticotropes to CRF may be due in part to the actions of these agents on CRF-R1 mRNA accumulation.

Activin-A inhibits proopiomelanocortin messenger RNA accumulation and adrenocorticotropin secretion of AtT20 cells

The complexity of corticotropic cell regulation by multiple central and peripheral factors is well recognized. The present study provides evidence for the participation of an additional factor in the regulation of this cell type of the anterior pituitary. Using the clonal AtT20 cell line as a model for corticotropes, homodimeric activin-A was observed to suppress basal ACTH secretion and POMC mRNA accumulation by approximately 50%. These effects required prolonged treatment with activin-A and were concentration dependent; the half-maximum concentration was in the range of 30-50 pM. Consistently, AtT20 cells were found to express specific high affinity binding sites for [125I]activin-A. The simultaneous addition of inhibin-A along with increasing concentrations of activin-A did not alter the characteristics of the inhibition of ACTH secretion by activin-A alone. This is in contrast to observations with gonadotropes of the anterior pituitary as well as a number of other cell types in which inhibin-A can partially antagonize the biological actions of activin-A. The results may suggest the participation of a subclass of activin receptors that mediate effects on ACTH secretion and POMC mRNA accumulation. As previously shown, the incubation of AtT20 cells with a synthetic glucocorticoid, dexamethasone, attenuated basal ACTH secretion and POMC expression in a concentration-dependent manner. The inhibition of both of these parameters by activin-A, however, was independent of glucocorticoids, because the two agents were additive in their actions. In addition to effects on secretion and mRNA levels, treatment with activin-A also inhibited the rate of proliferation of AtT20 cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular responses to gonadotropin-releasing hormone in a clonal cell line of the gonadotrope lineage

We recently derived a GnRH-responsive pituitary cell line of the gonadotrope lineage (alpha T3-1) by targeted oncogenesis in transgenic mice. Here, we report studies characterizing the GnRH receptors present in these cells and the intracellular responses to GnRH treatment. The receptors in alpha T3-1 cells show specificity for different GnRH analogs, with dissociation constants very similar to those found in normal rat and mouse pituitary. The concentration of receptors is within the range found in normal pituitary. The addition of GnRH or GnRH agonists increases phosphoinositide turnover and protein kinase-C translocation to membranes, and enhances activation of voltage-sensitive calcium channels. However, GnRH does not affect cAMP levels. Analysis of alpha-subunit mRNA levels demonstrated induction by GnRH and phorbol esters. Our results indicate that GnRH initiates a cascade of intracellular events that generate a set of second messengers, one or more of which is involved in the regulation of gene expression. The responses of alpha T3-1 cells to GnRH appear to have characteristics equivalent to those of primary pituitary gonadotropes, indicating the utility of this cell line as a model system for the study of GnRH responses.

The cellular actions of vasopressin on corticotrophs of the anterior pituitary: resistance to glucocorticoid action

The cellular actions of vasopressin (AVP) in the anterior pituitary were investigated. HPLC analysis of [3H]inositol-labeled cells indicated that AVP stimulated a rapid increase in inositol-1,4,5 trisphosphate (IP3), inositol-1,4 bisphosphate, and inositol-4 monophosphate levels. While CRF had no effect on basal IP3 levels, it blocked their stimulation by AVP. CRF-stimulated ACTH secretion and cAMP accumulation were potentiated by AVP. AFter dexamethasone (DEX) treatment (20 nM, 18 h), CRF-dependent ACTH secretion and cAMP accumulation were attenuated but AVP was still able to potentiate both of these actions of CRF suggesting that cellular actions of AVP may be resistant to DEX effects. Therefore, [3H]AVP binding was determined in control and DEX-treated cells. Pretreatment with DEX had no effect on either AVP receptor affinity or on the number of available binding sites. Consistently, stimulation of IP3 production by AVP in DEX-treated cells was comparable to that of control cells. Protein kinase C activators such as 12-O-tetradecanoyl-phorbol-13-acetate and dioctanoylglycerol were either near additive with CRF or also potentiated the action of CRF on ACTH secretion, respectively, even after DEX pretreatment. These results indicate that, in the anterior pituitary, distinct intracellular signaling pathways mediate the actions of CRF and AVP; cAMP mediates CRF actions and IP3/protein kinase C mediate the effects of AVP. Neuromodulation of ACTH secretion by dual effector mechanisms which exhibit a complex mode of interaction and only one of which is negatively influenced by glucocorticoids, provides these cells a mechanisms by which appropriate responses can be elicited under various physiological states.